1. Field of the Invention
The present invention relates to communications systems and a mobile system, and more particularly, to a wideband communications system and a mobile system with a variable load circuit whose impedance corresponds to a frequency division factor of a frequency synthesizer.
2. Description of the Prior Art
Generally speaking, when transmitting data in a communications system, the communications system (mobile system) will perform modulation on data pending transmission to form a corresponding pending transmission signal. Then, the communications system will utilize an amplifier circuit to perform power amplification on the pending transmission signal before sending the pending transmission signal out. Please refer to FIG. 1, which is a circuit diagram of a tuned amplifier 10 according to the prior art. The tuned amplifier 10 is biased from a direct current (DC) voltage V to ground G, and comprises an n-type metal-oxide-semiconductor (NMOS) transistor M0 that realizes the amplification function and is coupled to a load circuit 12 comprising a capacitor Cp and an inductor Lp that form an LC-tank, and a resistor Rp. The equivalent impedance of the load circuit 12 is dependent on the operating frequency of the tuned amplifier 10. The gate of the NMOS transistor M0 is utilized to receive an input signal Si, e.g. the pending transmission signal, and the drain of the NMOS transistor M0 is utilized to output an amplified output signal So that is generated by the current conducted between the drain and the source of the NMOS transistor M0 flowing through the load circuit 12.
Please refer to FIG. 2, which is a diagram of the equivalent impedance of the load circuit 12 in accordance with the operating frequency of the tuned amplifier 10. The horizontal axis in FIG. 2 represents frequency, and the vertical axis in FIG. 2 represents the magnitude of the equivalent impedance. The equivalent impedance provided by the LC-tank exhibits a bandpass characteristic over the frequency spectrum. The bandpass characteristic can be described by resonant frequency and quality factor Q. The resonant frequency decides the position of the bandpass characteristic in the frequency spectrum, whereas the quality factor Q reflects bandwidth and gain of the bandpass characteristic. Changing the capacitance value of the capacitor Cp and the inductance value of the inductor Lp changes the resonant frequency and the quality factor Q of the bandpass characteristic, likewise changing the bandpass characteristic over the frequency spectrum. FIG. 2 shows different bandpass characteristics resulting from different quality factors Q. As shown in FIG. 2, when the capacitor Cp and the inductor Lp of the LC-tank combine to give the load circuit 12 a relatively high quality factor Q, the gain of the equivalent impedance is also relatively high. On the other hand, the relatively high quality factor Q results in a relatively narrow bandwidth. Likewise, if the capacitor Cp and the inductor Lp of the LC-tank combine to form a relatively low quality factor Q, then the bandwidth of the bandpass characteristic will be relatively wide, but the impedance will be reduced. If the impedance of the load circuit 12 is low, then the overall power gain of the tuned amplifier 10 will be reduced, making the tuned amplifier 10 unsuitable for signal power amplification.
Please refer to FIG. 3, which is a diagram of different equivalent impedances provided by the load circuit 12 at different frequencies. The horizontal axis in FIG. 3 represents frequency, and the vertical axis in FIG. 3 represent equivalent impedance magnitude. In FIG. 3, assume that frequencies f0 and f1 individually represent two center frequencies of two different frequency channels, respectively, and that the power amplification provided by the tuned amplifier 10 at each frequency f0, f1 is determined by the equivalent impedances provided by the load circuit 12 at each frequency f0, f1. As shown in FIG. 3, if the capacitor Cp and the inductor Lp of the LC-tank combine to give the load circuit 12 a relatively high quality factor Q, the equivalent impedance of the load circuit 12 will have a narrower bandwidth. This represents that the bandpass characteristic of the equivalent impedance will have high frequency selectivity, and the equivalent impedances provided at different frequencies will be very different. In other words, if the load circuit 12 has a high quality factor Q, the load circuit 12 will be unable to provide similar equivalent impedance at the frequencies of different frequency channels. In turn, the tuned amplifier 10 will also be unable to provide similar power gain for the pending transmission signal Si for different frequency channels.
In contrast, if the capacitor Cp and the inductor Lp of the LC-tank combine to give the load circuit 12 a relatively low quality factor Q, the equivalent impedance of the load circuit 12 will have a wider bandwidth, meaning the frequency selectivity will be reduced. The equivalent impedance provided at different frequencies will also be more even. However, as shown in FIG. 2, the lower quality factor Q causes the equivalent impedance of the load circuit 12 to be lower, which is a disadvantage for signal power amplification.
In order to increase multiplexing efficiency of data transmission, different members of different network systems can use signals in different frequency channels to exchange data. For example, wireless networks under the IEEE 803.11 a standard separate the 1 GHz frequency band into multiple different frequency channels, and each frequency channel occupies a different frequency range. In order to satisfy the communications requirements of this multi-channel specification, amplifiers in mobile systems should be able to provide similar power gain across a wide range of channel frequencies for the pending transmission signals. However, the tuned amplifier 10 of the prior art is formed of the fixed-value capacitor Cp and the fixed-value inductor Lp, which causes a tradeoff between gain and bandwidth. If the combination of the capacitor Cp and the inductor Lp has a high quality factor Q, then the bandwidth will be sacrificed, which makes it impossible to provide similar power gain for the pending transmission signal at different channel frequencies. If the combination of the capacitor Cp and the inductor Lp has a low quality factor Q, then the gain will be sacrificed, and it will be impossible to provide optimum power gain. Thus, it is hard for the fixed-LC tank tuned amplifier of the prior art mobile system to meet both power gain and bandwidth requirements.